Many systems for production of three-dimensional modeling by photohardening have been proposed. European Patent Application No. 250,121 filed by Scitex Corp. Ltd. on June 6, 1987, provides a good summary of documents pertinent to this art area, including various approaches attributed to Hull, Kodama, and Herbert. Additional background is described in U.S. Pat. No. 4,752,498 issued to Fudim on June 21, 1988.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable polymer with a laser beam according to a desired pattern and building a three-dimensional model layer by layer.
However, all these approaches fail to identify practical ways of utilizing the advantages of vector scanning combined with means to maintain constant exposure and attain substantially constant final thickness of all hardened portions on each layer throughout the body of the rigid three-dimensional object. Furthermore, they fail to recognize very important interrelations within specific ranges of operation, which govern the process and the apparatus parameters in order to render them practical and useful. Such ranges are those of constant exposure levels dependent on the photohardening response of the material, those of minimum distance traveled by the beam at maximum acceleration dependent on the resolution and depth of photohardening, as well as those of maximum beam intensity depend on the photospeed of the photohardenable composition.
The Scitex patent, for example, suggests the use of photomasks or raster scanning for achieving uniform exposure, but does not suggest a solution for keeping the exposure constant in the case of vector scanning. The use of photomasks renders such techniques excessively time consuming and expensive. Raster scanning is also undesirable compared to vector scanning for a number of reasons, including:
necessity to scan the whole field even if the object to be produced is only a very small part of the total volume,
considerably increased amount of data to be stored in most cases,
overall more difficult manipulation of the stored data, and
the necessity to convert CAD-based vector data to raster data.
On the other hand, in the case of vector scanning only the areas corresponding to the shape of the rigid object have to be scanned, the amount of data to be stored is smaller the data can be manipulated more easily, and "more than 90% of the CAD based machines generate and utilize vector data" (Lasers & Optronics, Jan. 1989, Vol. 8, No. 1, pg. 56). The main reason why laser vector scanning has not been utilized extensively so far is the fact that, despite its advantages, it introduces problems related to the inertia of the optical members, such as mirrors, of the available deflection systems for the currently most convenient actinic radiation sources, such as lasers. Since these systems are electromechanical in nature, there is a finite acceleration involved in reaching any beam velocity. This unavoidable non-uniformity in velocity results in unacceptable thickness variations. Especially in the case of portions of layers having no immediate previous levels of exposure at the high intensity it becomes necessary to use high beam velocities, and therefore, longer acceleration times, which in turn result in thickness non-uniformity. The use of low intensity lasers does not provide a good solution since it makes production of a solid object excessively time consuming. In addition, the usefulness of vector scanning is further minimized unless at least the aforementioned depth and exposure level relationships are observed as evidenced under the Detailed Description of this invention.
No special attention has been paid so far to the composition itself by related art in the field of solid imaging, except in very general terms.
Thus, the compositions usually employed, present a number of different problems, major ones of which is excessive photohardening depthwise usually accompanied by inadequate photohardening widthwise and non-uniformity in thickness. These problems become especially severe in cantilevered or other areas of the rigid object, which areas are not immediately over a substrate. Another major problem has to do with shelf stability of the photohardenable composition, due to settling.
Therefore, it is an object of this invention to resolve the problems cited above by incorporating appropriate core or core-shell radiation deflecting matter in the photohardenable composition in order to limit the depth of photohardening with simultaneous increase of the width of photohardening, so that the resolution is better balanced in all directions, and at the same time the shelf stability improves considerably. The term "core-shell polymers" as discussed hereinafter is taken under its most broadest meaning, which also includes polymers consisting essentially of a core without a shell.
European Patent Application 250,121 (Scitex Corp., Ltd.) discloses a three-dimensional modelling apparatus using a solidifiable liquid which includes radiation transparent particles in order to reduce shrinkage.
U.S. Pat. No. 4,753,865 (Fryd et al.) describes solid photopolymerizable compositions containing addition polymerizable ethylenically unsaturated monomer, initiating system polymer, binder and microgel, wherein preferably the binder and microgel form substantially a single phase.